Combustion misfires lead to an increase of toxic substances emitted during operation of the engine and can, in addition, lead to damage of the catalytic converter in the exhaust-gas system of the engine. A detection of combustion misfires in the entire rpm and load ranges is necessary to satisfy statutory requirements as to onboard monitoring of exhaust-gas relevant functions. In this context, it is known that, during operation with combustion misfires, characteristic changes occur in the rpm curve of the engine compared to normal operation without misfires. Normal operation without misfires and operation with misfires can be distinguished from a comparison of these rpm curves.
A method operating on this basis is already known and disclosed in German patent publication 4,138,765 which corresponds to U.S. patent application Ser. No. 07/818,884, filed Jan. 10, 1992, now abandoned.
In this known method, a crankshaft angular region which is characterized as a segment is assigned to a specific region of the piston movement of each cylinder. The segments are realized, for example, by markings on a transducer wheel coupled to the crankshaft. The segment time in which the crankshaft passes through this angular region is dependent, inter alia, upon the energy converted in the combustion stroke. Misfires lead to an increase of the segment times detected in synchronism with the ignition. According to the known method, a criterion for the rough running of the engine is computed from the differences of the segment times.
Depending upon the type of computation, misfiring cylinders cause high or positive rough-running values and combusting cylinders cause low or negative rough-running values. Misfiring cylinders are differentiated from combusting cylinders by comparing these values to a threshold dependent upon load and rpm.
In the context of the above, U.S. Pat. No. 5,263,365 discloses that the threshold with which the misfires of individual cylinders are detected must be reduced when simultaneous misfires occur in several cylinders. According to this state of the art, the threshold value for the detection of individual misfiring cylinders is reduced in the case of multiple misfires independently of the number of misfiring cylinders by a fixed amount or factor.